[Paper Review] Noise in stimulated Raman scattering measurement: From basics to practice

Xavier Audier, Sandro Heuke+3 more|arXiv (Cornell University)|Oct 28, 2019

Spectroscopy Techniques in Biomedical and Chemical Research37 references77 citations

TL;DR

This paper provides a practical tutorial to characterize laser intensity noise in SRS systems, derives how noise limits SRS detection, and offers experimental methods to measure and optimize RIN for lock-in based SRS. It connects theoretical noise models to practical measurements using commercial lasers.

ABSTRACT

We revisit laser intensity noise in the context of stimulated Raman scattering (SRS), which has recently proved to be a key technique to provide label free images of chemical bonds in biological and medical samples. Contrary to most microscopy techniques, which detect a weak photon flux resulting from light matter interactions, SRS is a pump-probe scheme that works in the high flux regime and happens as a weak modulation ($10^{-4}-10^{-6}$) in a strong laser field. As a result, laser noise is a key issue in SRS detection. This practical tutorial provides the experimentalists with the tools required to assess the amount of noise and the ultimate SRS detection limit in a conventional lock-in-based SRS system. We first define the quantities that are relevant when discussing intensity noise, and illustrate them through a conventional model of light detection by a photodiode. Stimulated Raman Scattering is then introduced in its lock-in-based implementation, and the model presented is adapted in this particular case. The power spectral density (PSD), relative intensity noise (RIN), signal to noise ratio (SNR), and sensitivity of the system are derived and discussed. Two complementary methods are presented that allow measurement of the RIN and assessment of the performance of a SRS system. Such measurements are illustrated on two commercial laser systems. Finally, the consequences of noise in SRS are discussed, and future developments are suggested. The presentation is made simple enough for under-graduated, graduated students, and newcomers in the field of stimulated Raman, and more generally in pump-probe based schemes.

Motivation & Objective

• Define key noise quantities for light intensity detection and connect them to SRS detection limits.
• Adapt a conventional photodiode noise model to the lock-in based SRS setup.
• Derive PSD, RIN, SNR, and system sensitivity expressions relevant to SRS.
• Propose and illustrate two complementary RIN measurement methods on commercial lasers.
• Provide practical guidance to maximize SNR and discuss future noise reduction directions.

Proposed method

• Present a standard semi-classical model of photodetection with pulsed lasers.
• Derive PSD, RIN, and SNR expressions for a detector current under noise sources.
• Adapt the model to lock-in based SRS with amplitude modulation and demodulation.
• Derive the SNR and minimum detectable SRS gain, beta_min, in terms of RIN and system parameters.
• Describe two experimental procedures to measure laser RIN on commercial laser sources.
• Illustrate methods with measurements on two commercial laser systems.

Experimental results

Research questions

• RQ1 How does laser intensity noise (electronic, shot, and excess) limit SRS detection in a lock-in system?
• RQ2 How can RIN be measured and minimized to optimize SRS SNR and sensitivity?
• RQ3 What is the relationship between modulation frequency, lock-in bandwidth, and SRS detection limits?
• RQ4 How do different laser sources affect RIN and SRS performance in practice?

Key findings

• The SNR in a lock-in based SRS system scales inversely with the laser RIN integrated over the lock-in bandwidth.
• The SRS sensitivity beta_min is proportional to the square root of the product of lock-in bandwidth and RIN.
• Lock-in detection shifts low-frequency laser noise to the modulation frequency, enabling higher SNR by choosing f0 appropriately.
• Two experimental RIN measurement methods can characterize RIN surfaces over frequency and intensity ranges.
• Measured laser RIN on two commercial laser setups shows agreement with shot-noise limits at higher frequencies for one device, validating the model in practice.
• An explicit framework is provided to optimize SRS performance by balancing modulation frequency, bandwidth, and average detector current.